StatusNotFound is not enough for verifying the ErrNoSuchImage

[shiyuleixia]

if e, ok := err.(*Error); ok {
if e.Status == http.StatusNotFound {
return nil, ErrNoSuchImage
}

this part should also justify the exact error message ,such as this:
if e.Status == http.StatusNotFound && strings.Contains(e.Message, "No such image")

because it the network resource is not exist ,the docker daemon service also response StatusNotFound

[fsouza]

dup of #528, but thanks for reporting, I had forgotten about this.